In the semiconductor assembly and packaging industry, it is usually necessary to electrically connect different electronic devices, such as an integrated circuit chip or die to a substrate on which it is mounted by bonding. Such electrical connections can be done using conductive wires or direct connection between conductive pads. The bond pads on the electronic devices have to be aligned with respect to a bonding tool that is used for performing bonding.
For example, in wire bonding, the positions of the bond pads must first be determined prior to bonding conductive wires onto the bond pads. A vision system is typically used to capture images of an electronic device for alignment of its bond pads. The substrate to be bonded is placed onto a worktable, which is in turn mounted on a positioning stage such as an XY table. The positioning stage moves the electronic device relative to the vision system to capture images of predetermined points on the substrate for PR alignment. Sometimes, it is possible for the vision system to look for several position indicia on the electronic device, and use these to calculate the positions of all the bond pads on the electronic device if they are of a fixed pattern. In other cases, calculating bonding positions from selected position indicia may not offer enough precision, and each bonding position on each bond pad has to be recognized. One example is an LED device, wherein the bonding positions are scattered in dot matrix form over a substrate to be bonded, and each bonding position preferably should be individually recognized during PR alignment.
For bonding applications in general, the total bonding time would usually consist of: (1) loading and unloading of materials, (2) PR alignment, and (3) actual bonding. Where there are a large number of discrete bonding positions, such as for LED bonding, the PR alignment time occupies a significant portion of the total bonding time. Saving PR alignment time would be one major factor in improving the productivity of the bonding apparatus.
In terms of processing time, conventional PR alignment methods take up much time because a positioning stage needs to stop the electronic device at each bonding position relative to the vision system for camera exposure so that the vision system is able to capture images of the bonding positions to thereby align the bonding points. Thus, to move from point to point for PR alignment, the positioning stage needs to go through acceleration and deceleration for each change of position, not including the time spent when the electronic device is stationary. This method is time-consuming.
FIG. 1 is an illustration of a typical motion sequence showing the points at which PR image-grabbing is performed in a prior art PR alignment system. The motion velocity-time graph 10 approximates the motion of the positioning stage to position bonding positions of the electronic device relative to a vision system. The PR grab graph 12 indicates when an image of a bonding position is acquired. The graphs illustrate that at trough positions 14 when the positioning stage stops the electronic device at a bonding position, the vision system grabs an image of the bonding position at the same time 16. Therefore, the positioning stage needs to accelerate and decelerate in between adjacent bonding positions whereat the positioning stage is stopped for image-grabbing.
FIG. 2 is an illustration of a prior art PR alignment method wherein LED bonding positions 18 are moved relative to a vision system from point to point. It shows a row of LED bonding positions 18 which are regularly spaced apart. The positioning stage follows a motion sequence 20 to move from one LED bonding position 18 to another and stops at the position of each and every LED bonding position 18 for an image of the bonding position to be captured.
One way to reduce the effects of the long PR alignment time is to incorporate dual worktables holding separate electronic devices so that PR alignment can be performed on one electronic device while bonding is simultaneously being performed on another electronic device. However, such an approach is not cost-effective and may introduce control complexities in having to synchronize the worktables during simultaneous PR alignment and bonding.
The long exposure time required when using conventional PR alignment methods is a bottleneck during the PR alignment process. Since the wasted time that the electronic device is stationary is a major contributing factor to PR alignment time, whereas image-grabbing as such takes considerably less time, it would be advantageous to reduce the amount of time spent on accelerating and decelerating movement of the electronic device between bonding points. The table motion time is significantly longer when the table needs to stop at every point, as compared to when the table just passes through all the points at full speed. Accordingly, if PR alignment could be performed while avoiding the need to stop at each and every bonding position to capture an image of the bonding position, PR alignment time could be significantly reduced along with total processing time of the electronic device.